1. Field of the Invention
This invention relates to a wrench, more particularly to an electronic torque wrench having a trip unit.
2. Description of the Related Art
Conventional torque wrenches can be divided into mechanical types, such as those disclosed in U.S. Pat. Nos. 4,485,703, 5,129,293, and 5,435,190, and electronic types, such as those disclosed in U.S. Pat. Nos. 4,958,541, 6,981,436, and 6,968,759. Generally, a conventional mechanical torque wrench includes a tubular housing, a lever connected pivotally to the tubular housing and aligned with the same in a normal state, a ratchet drive head connected to the lever, and a compression spring for biasing the lever. When the torque applied by the wrench to a bolt is larger than a biasing force of the compression spring, the lever is displaced slantingly until it bumps against the tubular housing. As such, the user can clearly feel the trip made by the lever. However, a drawback of this kind of wrench is that it is difficult to accurately design the compression spring to provide a desired preset biasing force. Therefore, a proper biasing force cannot be provided, especially when the compression spring experiences fatigue.
A conventional electronic torque wrench generally employs a plurality of strain gauges secured to a lever to produce a variable resistance to thereby measure an applied torque. When the torque applied by the wrench exceeds a preset torque value, a processing unit of the wrench will activate a vibrating motor, an audible alarm signal, or an illuminating lamp to warn the user. Although the conventional electronic torque wrench can accurately set the preset torque value through an electronic control method, since the lever cannot be displaced so as to bump against the tubular housing, the user cannot directly and clearly feel the tripping of the lever, so that the user is likely to stop the operation too late, thereby resulting in applying excessive torque.